Tsunami shook entire planet, instruments show

Tsunami truly was Earth-shaking eventThe entire planet trembled — and did so for weeks, instruments show

RANDOLPH E. SCHMID, Associated Press

Published 5:30 am, Friday, May 20, 2005

WASHINGTON - The great Sumatra-Andaman earthquake that generated the devastating tsunami in December was so powerful that the ground shook everywhere on the Earth's surface and weeks later the planet still trembled.

Ground movement of at least 0.4 inch occurred everywhere as a result of the strongest quake in more than 40 years, though the sensation was not noticed in many areas. The quake resulted from the longest fault rupture ever observed — 720 miles to 780 miles, which spread for 10 minutes. A typical earthquake lasts for 30 seconds.

The quake was the first of its size to be measured and studied by the new worldwide array of digital seismic instruments.

A special section of a half-dozen research papers on the quake appears in today's issue of the journal Science.

The earthquake and resulting tsunami, which swept across the Indian Ocean, killed more than 176,000 people in 11 countries. About 50,000 people were left missing and hundreds of thousands homeless.

The quake occurred where two of the giant plates that form the surface of the Earth grind together.At that spot, the Eurasian plate was being pulled downward by the descending Indo-Australian plate. The quake released the edge of the Eurasian plate, which sprang up, lifting the ocean floor and sending the seawater off in the giant wave, the researchers reported.

They said the higher sea floor displaced so much water from the Bay of Bengal and the Andaman Sea that sea level worldwide was raised by 0.004 inch.

The temblor also "delivered a blow to our planet" that was felt for weeks, according to a team of researchers led by Jeffrey Park of Yale University.

His group calculated that the quake caused the planet to oscillate like a bell, at periods of about 17 minutes, which scientists were able to measure for weeks afterward.

The quake off Sumatra on Dec. 26 is estimated to have had a magnitude of 9.1 to 9.3. A second quake to the south on March 28 registered 8.6.

By comparison, the 1960 Chile earthquake was magnitude 9.5 and the one in Alaska in 1964 was 9.2. California's 1989 Loma Prieta earthquake had a magnitude of 6.9.